Between October and March, the British sun is a poor factory for vitamin D. Health officials already advise that everyone should consider a daily 10 microgram supplement through the darker months, with a stronger case for people with little outdoor time, darker skin, or diets low in oily fish and fortified foods.
Surveys still find a large minority walking into spring with low levels. Rickets is rare but not gone. More commonly, deficiency shows up as bone risk in older adults and as a marker of a life lived indoors.
A pill is not a personality, and it is not a substitute for a decent diet. It is, in this country, a seasonal piece of infrastructure, like putting the heating on. Food sources help — salmon, sardines, egg yolks, fortified milks — but they rarely close the winter gap alone.
Doctors warn against mega-doses bought on a whim. More is not better; vitamin D is stored in fat and can be toxic at high chronic intakes. The cheap supermarket tablet at the recommended dose is the unfashionable answer.
If you do one nutrition thing as the clocks change, this is a strong candidate. It will not make you an athlete. It may keep your bones in the game.